First observation of direct evidence for compact star formation in a supernova explosion sorae universe portal website

This happens at the end of the star's life when it is more massive than the Sun.Supernova explosion(Type II supernova), whose central core then collapses into a high-density “neutron star” or “black hole.”Compact starIt is known to be left behind. However, to date, there is no direct observational evidence that supernova explosions are associated with compact stars.

Two international research teams”SN 2022jliWhen observing a supernova explosion, they succeeded in capturing a unique light curve (change in brightness). This light curve is thought to be caused by a compact star born in a supernova explosion that inhales the expanding atmosphere of a companion star.The first observational record showing that supernova explosions and compact stars are linkedHe is.

■ There has been no direct evidence that a “compact star” survives a “supernova explosion.”

A “star”, like the sun, generates its own energy through nuclear fusion reactions that occur in its core, and avoids being crushed by gravity. However, at some point, nuclear fusion will stop, leading to a phenomenon called “gravitational collapse,” where the object collapses due to gravity.

For stars more than eight times the mass of the Sun, the core becomes a very dense mass due to gravitational collapse. Matter outside the nucleus later falls, collides with the nucleus, and bounces back. The details of the subsequent process are still not well understood, but it is possible that the elementary particles generated during the process of increasing the density of the central core are absorbed by the outer covering material, and that the particles that bounce off the central core and fall. Due to the enormous amount of energy generated when the particles collide with each other, Getting rid of particles other than the nucleus at high speed. This is it”Supernova explosion“He is.

If gravitational collapse stops in this truncated state, it becomes a “neutron star”, and if it cannot withstand gravity even in this state and gravitational collapse continues indefinitely, it becomes a “black hole”. Neutron stars and black holes are both much smaller in size, ranging from a few kilometers to several tens of kilometers in size, compared to stars with a diameter of several million kilometers, so they are collectively called “”Compact star」(※)it's called.

*Although not mentioned in this article, compact stars also include white dwarfs with a diameter of about 10,000 km. This arises from the core of a star that does not explode as a supernova, such as the Sun.

The formation of compact stars in conjunction with supernova explosions has been theoretically verified several times, and is strongly expected. There are also many cases where compact stars have been found following previous supernova explosions. A famous example of this is the Crab pulsar, a neutron star located at the center of the Crab Nebula, which is a supernova remnant created by a supernova explosion observed in 1054 AD.

Thus evidence is accumulating on the relationship between supernova explosions and compact stars, and the situation is beyond doubt. However, the “near real-time” detection of observational records indicating the presence of a compact star immediately after a supernova explosion has never been achieved. Normally, compact stars are thought to form within only a few seconds when a supernova explosion occurs, but because the site of the supernova explosion is surrounded by a huge amount of matter and radiation, the compact star located at the center is due to radiation from stars that are hidden and cannot be directly detected for long periods. from time.

If you want to be scientifically accurate, you could point out that the idea that compact stars form at the same time as supernova explosions is currently based on circumstantial evidence and there is no direct evidence. Therefore, finding a compact star immediately after a supernova explosion would require some other observational evidence.

■ What is the reason for the periodic brightness changes of “SN 2022jli”?

「SN 2022jliIt is a supernova explosion that occurred in the galaxy “NGC 157”, which is located about 75 million light-years away in the direction of the Cetus constellation as seen from Earth. This supernova explosion was first observed and studied by T. Moore of Queen's University Belfast, and further details were revealed by a research team led by Ping Chen of the Weizmann Institute of Science, which conducted additional observations. The two research teams' papers were published in the Astrophysical Journal Letters in 2023 and in Nature in 2024, respectively.

First, Moore and his colleagues used instruments such as the New Technology Telescope (NTT) installed at the La Silla Observatory in Chile to create a light curve showing how the brightness of SN 2022jli is changing. I get it. A typical supernova explosion produces a light curve that has an initial brightness peak and gradually fades over time.

However, the light curve of SN 2022jli was very unusual. SN 2022 jli started out brighter than a typical supernova, then dimmed for about 25 days before brightening again about 52 days after it exploded. Continuing observations over the next 200 days revealed that although it became progressively fainter,A cyclic light curve that becomes brighter and darker repeatedly every 12.4 days.It was received. SN 2022jli is the first time that the brightness of a supernova has been observed to change periodically over such a long period of time.

Although Moore and his colleagues could not determine exactly why SN 2022 jli's periodic brightness changes, they proposed two hypotheses. One theory is that it occurs when material ejected before the star explodes periodically collides with material ejected after the supernova explosion. Another reason is that the star that exploded as a supernova has a companion star.Energy was released when the merging star collided with the atmosphere of the companion star.This is the theory.

■ Obtain direct evidence of the existence of compact stars through changes in brightness

Chen and others used the X Shooter spectroscopic observation instrument on the Very Large Telescope (VLT) at the Paranal Observatory in Chile, and the LAT detector instrument of the Fermi Gamma-ray Space Telescope. We made more detailed observations of SN 2022 jli. As a result, spectral lines and gamma-ray emission indicating the presence of hydrogen were observed, and it was concluded that the latter theory was the most likely.

A more detailed explanation of the latter theorem is as follows. The compact star left behind after the supernova explosion continues to maintain a binary relationship with its companion star when it was a star. As a result of interaction with material ejected from the supernova explosion, this companion star expands more than usual, facilitating the peeling off of the hydrogen atmosphere on its surface. When a compact star orbits and approaches a companion star, it strips the atmosphere from the companion star's surface and forms an “accretion disk” around it. Electromagnetic waves are emitted by collisions between materials in the accretion disk, and this is thought to be related to bright radiation that occurs every 12.4 days.

In this observation, we were unable to detect direct radiation from the compact star. However, taking all the evidence obtained from observations together, the best explanation for this phenomenon is the presence of merging stars, and it is difficult to interpret all the observational evidence consistently with other assumptions. In other words, on this note,For the first time, direct observational evidence has been obtained that a compact star formed directly after a supernova explosion.it will be.

Although this research succeeded in obtaining direct evidence of the existence of compact stars, many mysteries remain. For example, it was not possible to directly observe the merging stars or explore their precise properties. We also don't know what kind of evolution or fate a binary star like SN 2022jli will have. Mysteries related to these properties could be solved by the European Extremely Large Telescope (E-ELT), which is scheduled to become operational in 2025.

source

• T. Moore, et al. “SN 2022jli: a Type Ic supernova with periodic modification of the light curve and an unusually long spike”. (The Astrophysical Journal Letters)
• Ping Chen, et al. “12.4-day periodicity in a nearby post-supernova binary system”. (nature)

Written by Riri Aya